I. Field of the Invention
The present invention relates to a method of improving the anti-inflammability of dimethyl silicone oil for use in static electric apparatuses.
II. Description of the Prior Art
Polychlorinated biphenyls (PCBs) have heretofore been used as insulating oils for transformers or as fire-retardant dielectric liquid for high-voltage capacitors. As PCBs cause environmental pollution exerting an adverse influence on human bodies, dimethyl silicone oils are currently used in place thereof. Dimethyl silicone oils exhibit superior heat resistance and favorable electrical insulating characteristics and they have a high flash point and a short fire duration. Therefore, they have superior properties as oils for use in static electric apparatuses. It is recognized that an open flash point of 300.degree. C. or higher is one of the important properties which fire-retardant dielectric liquids must have.
As further demand for fire retardation has been raised for static electric apparatuses, fire-retardant dielectric liquids or oils having higher flash points or slower fire propagation rates have been strongly desired.
It is to be noted that there is such a relationship between kinematic viscosities and flash points of dimethyl silicone oils as shown in Table A.
TABLE A ______________________________________ Kinematic viscosities and flash points of dimethyl silicone oils Kinematic Viscosity (cSt, 25.degree. C.) Flash Point (.degree.C.) ______________________________________ 5 149 20 210 50 308 100 330 350 349 1,000 343 ______________________________________
It appears from Table A that the elevation of the kinematic viscosity of dimethyl silicone oils will result in a higher flash point. It is not desired, however, to raise the kinematic viscosity of dimethyl silicone oils too high because such a high kinematic viscosity will adversely affect its cooling capability in static electric apparatuses and its impregnability into impregnable insulating base materials such as insulating paper to be used in static electric apparatuses.